Error when plugin installed with 1.6.8.3

Hi,
i get an error when i click on wp-admin/admin.php?page=awm-group

Fatal error: Class 'SoapClient' not found in
wp-content/plugins/advanced-access-manager/mvb_wp_access.php on line 147

http://ww.wp.xz.cn/extend/plugins/advanced-access-manager/